In recent years, society has become increasingly aware of the finite nature of the earth's natural resources, particularly the fossil fuels, coal, crude petroleum and natural gas. As these energy sources become continually more scarce, the cost of recovering them increases, and the price a consumer must pay for energy likewise increases. Consequently, consumers and industry alike are now looking at the more abundant and potentially less expensive alternative energy sources, such as wind, sunlight and geothermal energy production, to supply their energy needs.
Typically, the energy tapped from these alternative energy sources is used in the form of electricity, e.g., a solar array acting as a DC power source and a windmill providing AC power through a generator means. On the scale of an individual household, this electric power may be used to supply electrical appliances or stored in batteries for later use. However, to power even small household electrical appliances, the electric power supplied by alternative energy sources is often too intermittent and unpredictable in magnitude to use as the sole power source, primarily owing to the small scale of energy collecting devices employed and the fickle nature of local climatic conditions. Supplemental power may be obtained from storage batteries, but again the amount of power that is available from the batteries is uncertain. Consequently, alternative energy sources of the type mentioned above are usually employed as supplementary sources of energy, aiding the local electric utility in supplying power to a given load. This is particularly true where a constant and uniform supply of electricity is required by the load. Such a supplemental source of energy may be used independently of the electric utility by connecting the supplemental source alone to selected loads only when supplemental energy is available. Alternatively, the supplemental energy source may be adapted to supply power directly to the utility's power grid.
To supplement the power supplied by an electric utility, however, the voltage signal supplied by an alternative energy source must be converted to an AC signal having the frequency of the AC voltage signal supplied by the utility, e.g., 60 Hz. in the United States. This is done by first converting the raw voltage signal supplied by the alternative energy source to DC, if it was originally AC, and then using a DC-AC power converter to transform the DC signal into a usable AC voltage signal of the proper frequency. The need for this latter frequency conversion arises because of the construction of the appliances being supplied, which are often designed to operate most efficiently at the frequency of the AC voltage signal supplied by the utility grid.